Five component photographic objective of the modified gauss type



FIVE COMPONENT PHOTOGRAPHIC OBJECTIVE OF THE MODIFIED GAUSS TYPE GeorgeH. Aklin, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester,N.Y., a corporation of New Jersey Filed Dec. 10, 1957, Ser. No. 701,881

7 Claims. (Cl. 88-57) This invention relates to high aperturephotographic objectives of the modified Gauss type or M-type, that is,to a modification of the type of objective which comprises two negativemeniscus components concave toward each other and aligned between twopositive components.

The object of the invention is to provide a high aperture objective inthe range of about f/2.0 to about f/ 1.5 which is very highly correctedfor spherical aberration, axial and lateral color, distortion, curvatureof field, astigmatism, zonal spherical aberration, and oblique sphericalaberration and coma so as to give a very highly resolved image over amoderate field of view of about :10 to 15.

The object of a special form of the invention is to provide an objectivefor use in combination with a prism between the objective and the focalplane on the short conjugate side.

The standard or norm of the Gauss or M-type objective consists of fourcomponents of which the outer two are positive and the inner two arenegative meniscus components concave toward the central air space inwhich the aperture stop is mounted, at least one component on each sideof the central airspace being compound for achromatizing the image. Thisstandard type has been modified in various ways such as by splitting oneor both of the positive components into two positive components each, bymaking more than two of the components compound, by introducing a smallairspace into one or more of the components, or in other ways. It isgenerally -known in the art that increasing the number of elements orcomponents tends to improve a lens system, butit is not practical andeconomical to do this indiscriminately because of the added expense andalso because of the increase in the stray light due to reflection fromadditional glass-air surfaces, the latter reason however being not sopotent since the development of anti-reliection coatings. Generally theaim is to get as much in improved optical performance as possible forthe increased expense of manufacture.

According to the present invention, a photographic objective is made upcomprising five components in optical axial alignment and separated byair spaces, of which the front and rear components are positive andconvex to the outside, the second and third components from the frontare negative, meniscus and concave toward the diaphragm spacetherebetween, and the fourth component is numerically the weakest andconsists of a front positive element and a rear negative elementcemented together, the refractive index of the positive element beinggreater than that of the negative element by between 0.1 and 0.4 and theradius of curvature of the cemented surface being between 0.25 F and -F,where F is the focal length of the objective as a whole, and where theminus sign indicates that the surface is concave toward the front. Itwill be noted that the first, second, third, and fifth componentscorrespond in general to the first, second, third and fourth componentsof the normal or standard M-type lens, and that the modification lies inproviding an additional doublet of weak` power constituting the oppositesense in the rear member.

Patented Apr. 18, 1961 ice fourth component of the objective accordingto the invention. At least one component in front of the diapragm spaceand at least two behind the diaphragm space are compound forachromatizing the image.

Preferably the shape of said fourth component is such that the curvatureof its front surface is between -1/f and +0.3/f.

I have yfound that a known but unusual feature of the prior artcooperates favorably with the fourth component as above defined, inbalancing out zonal errors. According to this preferred feature thethird component is compound and consists of a front positive meniscuselement and a rear negative meniscus element, the refractive index ofthe negative element being higher than that of the positive element bybetween 0.01 and 0.05.

The compound components are usually cemented, but the use of a smallairspace or intemal contact surface as defined in Patent No. 2,433,438to Cox in one or two components is considered to be within the scope ofthe invention. In case a small airspace is included in a component, Iprefer the curvatures of the two facing surfaces to differ by less than0.25/1.

The powers and shapes of the first, second, third and fifth componentsare generally similar to those of the first, second, third and fourthcomponents respectively of known M-type objectives. tive components ofobjectives according to theinvention are preferably within the rangesdefined in the following table:

where P1 to P5 are the powers of the individual components and P is thepower of the objective as a whole.

The shapes of the individual components may conveniently -be defined bythe radius of curvature of the front surface of each component, and asso defined are preferably within the ranges set forth as follows:

where R1, R3, R6, R9,vand R12 are the radii of curvature t of therespective front surfaces of the individual components. The radius ofcurvature of the rear surface of each component is necessarily such asto give the component the required power.

The curvatures of the internal contact surfaces within the componentsare determined by the requirements of achromatism in a well-knownmanner. The general scheme is to consider the components in front of thediaphragm space as one member of the objective and the components behindthe diaphragm space as 'a second member.A In setting up a rough design,provisional values of the dispersive indices and the curvatures oftheinternal contact 'surfaces are determined by the well-known (D-d) methodor in some equivalent mannerto give at least approximate correction ofaxial color. The lateral color is next computed, and'it is changed asnecessary by changing the degree of color correction'of the front memberand restoring the axial color by a change in the Computing one suchchange gives the coefficient of change of lateral color as a function ofthe (D-d) value of the front member, and the amount of change to givecomplete correction is then computed directly. This function may not bestrictly linear but it is nearly enough linear to give a rapidly Therespective powers of the` converging solution whereby both axial andlateral color are brought within acceptable limits. In some instances achange in an airspaee may be used as a supplementary measure. Eachmember being positive in power, a member is changed in the direction ofover-correction of color either by choosing a glass with a higherV-number in a positive element or a glass with a lower V-number in anegative element or by changing the radius of curvature of an internalcontact surface between two elements in the direction which algebraiclyincreases thepower of the element having the higher V-number anddecreases the power of the other element. The opposite changes, ofcourse, change the member in the direction of greater undercorrectionfor color. Many designers prefer to make the changes which improve axialand lateral color concurrently with bendings that are made for bringingthe monochromatic abberations closer to acceptable values rather thanmaking these changes separately as implied above. This, however, doesnot alter the principles of the method described above by which theradii of curvature of the internal contact surfaces are determined.

When a prism is used between an objective and its short conjugate focalplane, the objective itself should be undercorrected for sphericalaberration and color substantially as described in Patent No. 918,147Hastings, and may also be given a small residual of pincushiondistortion to compensate for the eiect of the prism. The resultingdesign problem is not greatly different from the designing of objectivesfor ordinary use, the chief difference being that the elect of the prismto be used is rst computed, and then in designing the objective Ithedesigner does not aim at the usual substantially zero residuals of thevarious aberrations but aims at residuals which will compensate for theeiect of the prism. The prism has very little eifect on the higher orderaberrations such as zonal spherical aberration and zonal astigmatism,and it has slightly more overcorrecting effect on the oblique sphericalaberration for a given aperture than at the axis. In practice it isusually suiiicient for adapting an objective for use with a prism tobend the several components to a degree determined customarily by aseries of solutions assuming linear coefficients of change of eachaberration with the bending of each component. This is a very commonmethod of design of lens systems, once the preliminary setup has beenmade.

The added component according to the invention has a very beneficialefrect on the zonal spherical aberration and on the oblique sphericalaberration because the marginal rays, which are ordinarilyovercorrected, strike the `collective cemented surface at a high angleof incidence and are disproportionately refracted. This, of course,tends to undercorrect the spherical aberration over the whole aperturebut this effect is easily counterbalanced by bending the variouscomponents during the trial solutions so that the whole sphericalaberration curve (asv graphed in the traditional way) is brought back tothe desired position (usually substantially vertical if there is to beno prism behind the objective). K

The preferred feature of structure of the third component provides aninternal contact surface which the rays also strike at a high angle ofincidence and at which the refractive index differs by a smaller amountand in the opposite sense from that at the surface in the fourthcomponent, and accordingly this surface in the third component isparticularly useful in conjunction with the added component according tothe invention for adding a final touch to the zonal spherical aberrationand the oblique spherical aberration correction.

In the accompanying drawing:

Figure l is a diagrammatic axial section of an objective according tothe invention for use with a prism between the objective and the focalplane.

Figure 2 is a table of constructional data for a'specic exampleaccording to Figure 1.

Figure 3 shows an objective according to the invention for use without aprism. j r Figure 1 shows an objective according to the invention madeup of five components. The first component is a single element orsinglet 1 of positive power with its more strongly curved surface to thefront. The sec-- 0nd component is a negative meniscus cemented doubletconsisting of a positive element 2 and a negative element 3. These firsttwo components differ only in detail from the prior art. The thirdcomponent is a negative meniscus doublet concave to the front,consisting of a positive meniscus element 4 and a negative meniscuselement 5 cemented thereto. 'Ihe diaphragm space in which the diaphragmor aperture stop is customarily mounted is between the second and thirdcomponents, and both these components are concavethereto, as ischaracteristic of this type of lens. The fourth component, in which theprincipal feature of the invention is embodied, consists of a positiveelement 6 having a relatively high refractive index and a negativeelement 7 having a refractive index between 0.1 and 0.4 lower than thatof the positive element cemented to the back thereof, the cementedsurface being concave toward the front and having positive power and aradius of curvature R10 between -F/4 and F. The lifth component, likethe first, is a positive singlet 8 in this example and is bi-convex inshape, as is usual in objectives of this type designed for use with avery distant object. In some M-type objectives for use at low or unitmagniication the rear component is plano-convex or even meniscus inShape, and the invention may advantageously be embodied in suchobjectives as well. Between the rear element 8 and the focal plane 10 isa prism 9 which is required in the particular application for which thisexample was designed. Prisms are used behind photographic objectives fora number of purposes exa ple as beam splitters in color photography orto reverse e image w en copying printed mater. It 1s well known 1n op caeory t at t e e e' t e prism upon the aberrations is the same as that ofa plane parallel plate having lthe same refractive index and opticalthickness. Accordingly, in Fig. 1, the prism is represented as aplane-parallel plate for simplicity.

Figure 2 is a table giving constructional data for a specific example ofthe embodiment of the invention shown in Figure 1.

This table is repeated here for convenience:

F=100mm. F:l.57

Lens N V Radll, mm. Thlcknesses,

R1 75.54 1 1.7767 44.7 l tr=| 9.33

s1= 1.83 Rz +5349 2 1. 7445 45.8 ti=14.39

R4 =+347.9 3 1.6725 32.3 lz= 4.96

sz=l5.89 Re 50.01 4 1.5880 61.2 4= 9.93 R1 -31.77

.1|= 1.42 Re =426.1 1. 8804 41. 1 ts=10.92

R1o= 52.46 1. 7200 29.3 t1,=, 7.49

RIF-440.1

8A=l 1.58 Riz=+126.3 v 8 1.7530 50.6 v tg=10.32

Rxs=-139.7

85=1500 Ru=w 9 1.5170 64.5 t=56.84

. Ris= front to rear; the second and third columns give the refractiveindex N for the sodium D line of the spectrum and the conventionaldispersive index V for each element, and the third and fourth columnsgive the radii of curvature R of the optical surfaces, the thicknesses tof the lens elements and the spaces s between the components, eachnumbered by subscript in order from front to rear. The space x5 betweenthe rear lens element 8 and the prism 9 is only nominal, because as iswell known the prism might be moved closer to the lens or farther fromit and its effect on the optical image would not be changed, thus, theprism may be mounted at the most convenient position for any particularenvironment in which this system is used.

In this example the index differences (N5-Nq) and (N5-N4) are 0.160 and0.029 in accordance with features of the invention described above.

The power of the respective components are as follows:

These powers are computed by the thin lens formula, disregarding theeffect of the thicknesses of the lens elements. I find that the effectof the thicknesses of the lens components does not vary excessively indifferent objectives of the type to which the invention pertains andthat this is a convenient way of defining the invention. It will benoted that these powers lie within the preferred ranges defined above,as do the radii R1, R3, Re, R9, R10, and R12, and that the fourthcomponent is numerically the weakest of the live.

As is well known, it is advantageous in correcting the astigmatism andthe zonal curvature of field to use high refractive index glasses,particularly in the positive elements. In this example the refractiveindices of all the elements are between 1.55 and 1.95 andthe averageindex of the positive elements is greater than 1.65, a feature whichcooperates with the invention to give better correction.

An objective made up exactly as specified in Figure 2 could be used in acamera in which there is no prism by providing a plane parallel platebetween the lens and the focal plane which would correspond in itseffect on the image to the prism shown in Figures 1 and 2. However, asthis adds unnecessary weight and expense, I contemplate modifying thedesign for use without a prism. To indicate roughly the probabledirection and degree of bendings of the components in such a redesign, Ihave made an approximate solution using rthe same coefficients of changeas were computed and used indetermining the radii given in Figure 2.This solution is set forth in the following table:

On Axis 7% R# Change (Fai/R) Aberr. Corna AF AY" AYL AY,l

81.. Final In this table the first column designates which radius orradii R or space s is changed, the second column indicates how much eachis changed (the space s measured in mm. and the radii in thedirnensionless ratio (FAI/R) where Al/R is (l/new R-l/old R).

In every case R18 is changed as necessary to maintain the E.F. unchangedat mm. The remaining columns of the table give the corresponding changesin the various aberrations indicated at the tops of the columns. Theseaberrations will be defined in the next paragraph below. These changesadded onto the Original values afthe top of the table give the estimatedFinal values at the bottom. The color aberrations remain to be-corrected by substituting a lower dispersion glass such as EK-330(l.755l/47.2) or the same glass as in element 8 (EK 230) in element 1 or2 or by varying the cemented surface R4 or both.

The aberrations in the table are defined as follows.

Ahern is spherical aberration'and is the intersection length of amarginal ray minus the back focal length. Coma is AF-Aberr. where AF is(Y1/sin U')F. Y1 is the semidiameter of the entrance pupil, and U' isthe inclination of an emerging ray which entered at the margin of theentrance pupil and parallel to the axis. AF' is the primary curvature offield computed along a ray entering the lens at the stated angle of 71/zand approximately through the center ofV the exit pupil. AYpt is thedistortion computed for the same ray. Finally AYL and AYu are thedistortion values for rays at or near the lower and upper margins of thepupil, assuming that the oblique pencil approaches the front of the objective from below the axis. These last twovalues are useful i-ndetermining the oblique spherical aberration and coma.

By means of the above coefficients, the primary monochromaticaberrations are corrected by the standard method of repeatedapproximations, and by standard methods outlined above the axial andlateral color are corrected whereby the example given is modified foruse as contemplated in the more usual environment without a prism. Theabove first step in this modication indicatesv that the radii would beroughly as follows: R1=68 mm., R2=195, R3=52, R5=30, R6=48, R9=85,R10=-57, R11=44, R11=j99 with further changes in the several radii, andparticularly in R1, to accommodate the change in refractive index in thefirst element, and R13 is solved for to give a focal length of 100 mm.

Figure 3, shows an objective in which the invention is applied to adifferent known form of the Gauss type or M-type objective. In thisknown form the first and third components are compound, consisting ofelements 31, 32 and 34, 3S respectively, and the second and lastcomponents are single elements 33 and 38. The invention is embodied inthe objective by the addition of the weak doublet consisting of elements36 and 37, corresponding to elements 6 and 7 as describedI above andbeing constructed within the ranges of power, shape and refractive indexaccording to the invention asset forth above and in the claims below.

The third component is compounded in the more ordinary way with thenegative element 34 in front of the positive element 35, to illustratethis form of the invention. However, it may also be constructed in theless usual reverse order shown by elements 4, S in Fig. 1.

The invention is similarly applied to other known forms of the M-typeobjective within the scope of the appended air surfaces as some priorknown lenses and give improved results in that the zonal sphericalaberration is almost perfectly corrected and the correction of sphericalaberration remains good over the designated field of view so that theresultant image is formed very sharply and gives very high resolution ofdetail, approaching the theoretical limit. over this field of view.

I claim:

1. An optical objective for photographic purposes comprising five lenscomponents in optical alignment, their dioptric powers P1 to P5,numbered from front to rear, being within the ranges defined as follows:

where P is the dioptric power of the objective as a whole, in which thefirst and fifth components are convex to the outside, the iirstcomponent has its more strongly curved surface to the outside, thesecond and third components are meniscus in shape and concave toward thediaphragm space therebetween, the fourth component is numerically theweakest component of the ve and consists of a front positive element anda rear negative element cemented together, the refractive index of thepositive element being greater than that of the negative element bybewteen 0.1 and 0.4 and the radius of curvature of the cemented surfacebeing between -0.25 F and -F, where F is the focal length of theobjective as a whole and where the minus sign indicates that the surfaceis concave to the front, and in which at least one component in front ofthe diaphragm space and at least one component behind the diaphragmspace in addition to said fourth component are compound forachromatizing the image.

2. An objective according to claim 1 in which the shape of said fourthcomponent is such that the curvature of its front surface is between-1/F and +0.3/F.

3. An objective according `to claim 2 in which the third component fromthe front consists of a front positive meniscus element and a rearnegative meniscus element, the refractive index of the positive elementbeing less than that of the negative element by between 0.01 and 0.05.

4. An objective according to claim l in which the third component fromthe front consists of a front positive meniscus element and a rearnegative meniscus element, the refractive index of the positive elementbeing less than that ofthe negative element by between 0.01

and 0.05.

5. An objective according to claim 1 in which the curvature R1, R3, R8,R9, and R13 of the respective front surfaces of said components are sochosen that the following algebraic inequalities hold:

'shapes of the said components as dened by the radii of v 8 6. Anobjective made substantially according to the following specications:

Lens N Radil Thlcknesses R1 =|0.8 F 1 1. 78 t1=0.09 F

Rz =|3 F s1=0.02 F R3 =+0.5 F 2 l. 74 tr=0.14 F

R4 =+3 F 3 1.67 t3=0.5 F

Rs =+0.3 F

sz=0.16 F Re =(J.5 F 4 1. 59 4=0.10 F

R1 =0.3 F 5 1. 62 i5=0.04 F

Rg=0.9 F

sa=0.01 F R9 =4 F G 1. 88 la=0.ll F

Rm=0.5 F 7 1. 72 t1=0.07 F

RIF-0.4 F

a4=002 F R13=+L3 F 8 1. 52 i8=0.l0 F

R1a=-1.4 F

where the lens elements are numbered in order from front to rear in therst column, where the corresponding refractivev index `for the Dspectral line is given in the second column, where the radii ofcurvature R of the optical surfaces, the thicknesses t of the lenselements and the axial separations s between the lens components, eachnumbered by subscripts from front to rear, are given in the last twocolumns, and where F is the equivalent focal length of the objective.

7. In combination, an objective according to claim 5 and a rism o ticallali ed therebehind between the objective and tlle image plane, saidprism Having an optical axial thickness between 0.4f and 0.7i.

France Aug. 12, 1957

